Previously, the tristate driver and the control signal generating network therefor were formed on separate integrated circuit chips. Testing of the tristate driver and the control signal generating network could then be performed separately. Hcwever, in state of the art large scale integrated circuits, the control signal generating network and the tristate driver are on the same integrated circuit chip. In such an integrated circuit chip, the control signal generating network can no longer be fully tested, because various test signal patterns applied to the control signal generating network will cause the tristate driver to assume its high impedance state. In the high impedance state however, the response of the tristate driver is no longer available so that the control signal generating network is no longer fully testable.